1. Field of the Invention
The present invention pertains to a method and system for obtaining seismic data pertaining to an earth formation by measuring certain vibrations generated by a drillbit or other device attached to a drillstring. Vibrations sensed by an array of seismic sensors on the earth's surface are compared with vibrations sensed by an arrangement of accelerometers and strain gages mounted at the upper end of the drillstring.
2. Background
U.S. Pat. No. 4,207,619 to A. Klaveness describes a technique wherein a pulse generator is attached to a drillstring and is used to generate seismic pulses at a selected depth in a wellbore, which pulses are measured as reflected and refracted vibrations by a series of seismometers mounted on the earth's surface. This technique, sometimes known as vertical seismic profiling, is considered useful in enabling the drilling operation to "look ahead" of the drillbit while drilling to gather information regarding the lithology of a formation in the vicinity of the wellbore.
The system described in the Klaveness patent does, however, require that the drilling operation be stopped while the seismic pulses are generated and measured by a sensor at the top of the drillstring as well as by the array of seismic sensors which detect the pulse signal which is transmitted as refracted and reflected acoustic or stress waves through the earth formation. Moreover, in the system described in the Klaveness patent only compressional seismic waves may be detected by the array and compared with the pulse generator output. It is also known to measure rock properties and drilling tool wear by sensing drillstem vibrations such as described in U.S. Pat. Nos. 3,520,375 to Raynal and 3,714,822 to Lutz.
However, with the development of the drillstring vibration measurement system described in U.S. Pat. No. 4,715,451 to A. A. Bseisu et al and assigned to the assignee of the present invention, an improved method of determining certain characteristics of an earth formation has been developed wherein the seismic signals generated by the drilling operation itself may be utilized in measuring the characteristics of a formation by the so called vertical seismic profiling technique. The improved system and method of the present invention has certain advantages which will be apparent to those skilled in the art upon reading the following summary and detailed description of the invention.